Depends on your intentions. If all you want is a rifle to play with at the range and shoot targets for the fun of it most any AR15 is going to be capable of doing that and working without giving you much heartache. DPMS, Bushmaster, Olympic Arms, Stag Arms, RRA are all fine plinkers. However if you want something that you will possibly be using for home defense or shooting competitions I would recommend a Colt. There are also other high-end brands like Daniel Defense, and Lewis Machine and Tool but before the panic buying you could walk into Wal-mart and pick up a colt M4gery for ~$1600 which was only $300 more than an equivalent Bushmaster.
This is a good read before you buy. http://www.m4carbine.net/showthread.php?t=7376
And a lot more if you really want to be informed. http://m4carbine.net/showthread.php?t=7009
Also, piston-driven AR's were all the rage for about 5 minuets not long ago and there are still loads of piston models out there. Most in the know will tell you that the piston system on an AR is over rated and unnecessary, not to mention not standardized. The original direct impingement system works just fine so stick with that.